being also severally helped by one another in their greatest wants: for when the moon is in conjunction with the sun, and her upper part receives all the light, then her lowez hemisphere (which would otherwise be altogether dark) is enlightened by the reflection of the sun-beams from the earth. When these two planets are in opposition, then that part of the earth which could not receive any light from the sun-beams, is most enlightened by the moon, being then in her full; and as she doth most illuminate the earth when the sun-beams cannot, so the grateful earth returns to her as great (nay greater) light when she most wants it; so that always that visible part of the moon which receives nothing from the sun, is enlightened by the earth, as is proved by Galilæus, with many more arguments, in that treatise which he calls Systema Mundi. True indeed, when the moon comes to a quartile, then you can neither discern this light; nor yet the darker part of her body; and that for a double reason;

1. Because the nearer it comes to the full, the less light does it receive from the earth, whose illumination does always decrease in the same proportion as the moon does increase.

2. Because of the exuberancy of the light in the other parts. Quippe illustratum medium speciem recipit valentiorem*. The clearer brightness involves the weaker; it being with the species of sight, as it is with those of sound; and as the greater noise drowns the less, so the brighter object hides that which is more obscure. But as they do always in their mutual vicissitudes participate of one another's light so also do they partake of the same defects and 'darkenings; for when our moon is eclipsed, then is their sun darkened; and when our sun is eclipsed, then is their moon deprived of its light, as you may see affirmed by Meslin +. Quod si terram nobis ex alto liceret intueri, quemadmodum deficientem lunam ex longinquo spectare possumus, videremus tempore eclipsis solis terræ aliquan

*Scal. exerc. 62.

† Epit. Astr. 1. 4. part 2.

partem lumine solis deficere, eodem plane mode sicut ex opposito luna deficit." If we might behold this globe of "earth at the same distance as we do the moon in her "defect, we might discern some part of it darkened in "the sun's eclipses, just so as the moon is in hers." For as our moon is eclipsed by the interposition of our earth, so is their moon eclipsed by the interposition of theirs. The manner of this mutual illumination betwixt these two you may plainly discern in this figure following.

Where A represents the sun, B the earth, and C the

moon: Now suppose the moon C to be in a sextile of increase, when there is only one small part of her body enlightened, then the earth B will have such a part of its visible hemisphere darkened, as is proportionable to that part of the moon which is enlightened; and as for so much of the moon, as the sun-beams cannot reach unto, it re

ceives light from a proportionable part of the earth which shines upon it, as you may plainly perceive by the figure.

You see then that agreement and similitude which there is betwixt our earth and the moon. Now the greatest difference which makes them unlike, is this, that the moon enlightens our earth round about, whereas our earth gives light only to that hemisphere of the moon which is visible unto us; as may be certainly gathered from the constant appearance of the same spots, which could not thus come to pass, if the moon had such a diurnal motion about its own axis as perhaps our earth hath. And though some suppose her to move in an epicycle, yet this doth not so turn her body round, that we may discern both hemispheres; for according to that hypothesis (say they) the motion of her eccentric doth turn her face towards us, as much as the other doth from us.

But now, if any question what they do for a moon, who live in the upper part of her body? I answer, The solving of this, is the most uncertain and difficult thing that I know of, concerning this whole matter. But yet unto me this seems a probable conjecture.

That the upper hemisphere of the moon doth receive a sufficient light from those planets about it; and amongst these, Venus (it may be) bestows a more especial brightness, since Galilæus hath plainly discerned that she suffers the same increases and decreases, as the moon hath; and it is probable that this may be perceived there, without the help of a glass, because they are far nearer it than we. When Venus (saith Keplar) lies down in the perige or lower part of her supposed epicycle, then is she in conjunction with her husband the sun; from whom, after she hath departed for the space of ten months, she gets plenum uterum, and is in the full.

But you will reply, though Venus may bestow some light when she is over the moon, and in conjunction, yet being in opposition, she is not visible to them, and what shall they then do for light?

I answer; then they have none; nor doth this make so great a difference betwixt those two hemispheres, as there is with us betwixt the places under the poles and the line. And besides, it is considerable that there are two kind of planets.

1. Primary; such whose proper circle do encompass the body of the Sun, whereof there are six; Saturn, Jupiter, Mars, Ceres or the Earth, Venus, Mercury. in the frontispiece.

As

2. Secondary; such whose proper circles are not about the sun, but some of the other primary planets. Thus are there two about Saturn, four about Jupiter, and thus likewise does the moon encompass our earth. Now it is probable that these lesser secondary planets, are not so accommodated with all conveniencies of habitation, as the others that are more principal.

But it may seem a very difficult thing to conceive, how so gross and dark a body as our earth, should yield such a clear light as proceeds from the moon; and therefore the Cardinal de Cusa* (who thinks every star to be a several world) is of opinion, that the light of the sun is not able to make them appear so bright; but the reason of their shining is, because we behold them at a great distance through their regions of fire, which do set a shining lustre upon those bodies that of themselves are dark. Unde si quis esset extra regionem ignis, terra ista in circumferentia sua regionis per medium ignis lucida stella appareret. "So that if a man were beyond the region of fire, this "earth would appear through that as a bright star." But if this were the only reason, then would the moon be freed from such increases and decreases, as she is now liable unto.

Keplar thinks that our earth receives that light whereby it shines, from the sun; but this (saith he) is not such an intended clear brightness as the moon is capable of, and therefore he guesses that the earth there is of a more

* De doct. ig. 1. 2. c. 12,

choaky soil, like the isle of Crete, and so is better able to reflect a stronger light; whereas our earth must supply this intention with the quantity of its body. But this I conceive to be a needless conjecture, since our earth (if all things were well considered) will be found able enough to reflect as great a light. For,

1. Consider its opacity; if you mark these sublunary things, you shall perceive that amongst them, those that are most perspicuous, are not so well able to reverberate the sun-beams, as the thicker bodies. The rays pass singly through a diaphanous matter, but in an opacous substance they are doubled in their return, and multiplied by reflection. Now if the moon and the other planets can shine so clearly by beating back the sun-beams, why may not the earth also shine as well, which agrees with them in the cause of this brightness, their opacity?

2. Consider what a clear light we may discern reflected from the earth in the midst of summer; and withal, conceive how much greater that must be which is under the line, where the rays are more directly and strongly reverberated.

3. It is considerable, that though the moon does in the night-time seem to be of so clear a brightness, yet when we look upon it in the day, it appears like some little whitish cloud: not but that at both times, she is of an equal light in herself. The reason of this difference is, because in the night we look upon it through a dark and obscure medium, there being no other enlightened body, whose brightness may abate from this: whereas in the day-time, the whole heavens round about it are of an equal clearness, and so make it to appear with a weaker light. Now because we cannot see how the enlightened parts of our earth do look in the night, therefore in com puing it with the moon, we must not consider her, as she is beheld through the advantage of a dark medium, but as she seems in the day-time. Now in any clear sunshine day, our earth does appear as bright as the moon, which at the same time does seem like some duskish cloud